Liftoff·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Had a technical phone screen for a software engineer role at Liftoff. The whole thing was basically one meaty coding problem around IPv6 parsing, which sounds niche but ended up touching a bunch of edge cases I didn't think about until I was already halfway through my solution.

Questions Asked (1)

Write a function that takes a string and, if it's a valid IPv6 address, returns the fully expanded canonical form with exactly 8 four-digit lowercase hex groups separated by colons. If it's not valid, return "Invalid".

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just split on ":" and pad each piece, and that worked fine for the simple cases.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

First clarify the rules for a valid IPv6 address, including the use of '::' for zero compression and the maximum of 8 groups. Then design a function that splits the input, validates each group, expands '::' to the correct number of zero groups, and formats each group to 4 hex digits. Finally, handle edge cases like multiple '::', invalid characters, and group count.

Pro tip: Mention that you would use a regex or a state machine to validate the address before expansion, and discuss the trade-offs between simplicity and performance. Also, note that you should test with edge cases like '::', '::1', and '2001:db8::1'.

1. Clarify requirements and edge cases

Ask if the input can have leading/trailing spaces, uppercase letters, or embedded IPv4 (e.g., ::ffff:192.0.2.128). Confirm that '::' can appear only once and that groups must be 1-4 hex digits.

2. Validate the address structure

Check that the string contains only hex digits, colons, and at most one '::'. Ensure the total number of groups (counting '::' as a placeholder for one or more zero groups) does not exceed 8.

3. Expand '::' and normalize groups

If '::' is present, split the string into left and right parts, count the existing groups, and insert the correct number of '0000' groups in between. Then pad each group with leading zeros to make it 4 digits.

4. Format and return the result

Join the 8 groups with colons and convert to lowercase. If any validation fails, return 'Invalid'.

Key Points to Mention

IPv6 address format: 8 groups of 4 hex digits, separated by colons.
Zero compression with '::' can appear only once and must represent at least one group of zeros.
Validation of hex digits (0-9, a-f, A-F) and group length (1-4 digits).
Handling of edge cases: '::', '::1', full address, invalid characters, too many groups.
Time and space complexity: O(n) where n is the length of the string.
Potential use of regular expressions for validation, but consider performance and readability trade-offs.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.